Kiyoshi inoue



Sept. 20, 1966 KIYO$H| {NOUE Re. 26,090

[ELECTRIC POWER SUPPLY APPARATUS FOR ELECTRIC] METHOD OF ELECTRICALDISCHARGE MACHINING Original Filed Aug. 29. 1960 2 Sheets-Sheet l INVENTOR. K/YOS'H/ nvauE BY M {W ,4 r rO/PMEVS Sept. 20, 1966 KIYOSHI INOUE26,090

[ELECTRIC POWER SUPPLY APPARATUS FOR ELECTRIC] METHOD OF ELECTRICALDISCHARGE MACHINING Original Filed Aug. 29, 1960 2 Sheets-Sheet 2INVENTOR. K/ rasw/ mam;

United States Patent 26,090 [ELECTRIC POWER SUPPLY APPARATUS FORELECTRIC] METHOD OF ELECTRICAL DIS- CHARGE MACHINING Kiyoshi Inoue, 1823-cl10me Setagaya-ku, Tamagawayoga-machi, Tokyo-to, Japan Original No.3,087,044, dated Apr. 23, 1963, Ser. No. 52,395, Aug. 29, 1960.Application for reissue Apr. 23, 1965, Ser. No. 450,579

12 Claims. (Cl. 21969) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to electric power supply apparatus forspark-discharge machining, and more particularly it relates to a new andimproved power supply apparatus for electric discharge machining whereinsecondary, follow-up discharging is effected after primary, maindischarging, and both the machining speed and the smoothness of thesurface of the work piece are improved.

The mechanism of the process of spark discharge machining is understoodto be as follows: As the distance between the electrode and work pieceis reduced, the flow of electrons which travel from the electrode towardthe work piece excites the liquid interposed between the electrode andwork piece, cumulatively produces a stream of electrons, and transformsit into spark discharge. Then, when the discharge path is in acompletely disintegrated state, the discharge points of the electrodeand the work piece evaporate because of the discharge heat and separateinto gaseous phase and molten liquid phase. Meanwhile, however, anelectromagnetic force (pinch effect) and a high-pressure mechanicalforce are created in the discharge region by the action of the dischargecurrent. Said forces act to form a crater in the liquid phase portionsand simultaneously expel the cut chips out of the discharge region. Whenthe discharging thus ends, the said crater thereafter assumes a dishshape, and in accompaniment, a creator mound is formed as a ring aboutthe periphery of said crater.

Since the electrode and work piece, once their crater mound and craterhave been formed, are separated by an infinitesimal gap, localshort-circuit points are ordinarily created at the crater mound portion.If, as a supposition, the discharging is not completed by the time thesaid short-circuit points are created, the residual energy will passthrough the short-circuit points in a concentrated manner and willinstantaneously melt and vaporize the said points.

When this phenomenon is viewed from the point of view of metal working,it may be correctly inferred that the process comprising the initialforming of the crater mound and crater followed by the melting of theshortcircuit points means that the process comprising the rough workingprocess of forming the crater mound and crater and the finishing processof melting away the short-circuit points can he doubly achieved within asingle discharge, thus multiplying the machining speed, and simplifyingthe process.

For the above reason, if an electric power source having a wide pulsewidth is used so that the discharge current will flow, unchanged, duringalso the short-circuiting of the crater mound, the aforesaid two kindsof processes can be made possible with a single discharge. On the otherhand, however, excessive discharging energy, in general, entails to aconsiderable degree the possibility of converting of the sparkdischarging into arc discharging. In spark discharge processing,transformation into arc discharging means impossibility of furtherprocessing. Therefore, in order to maintain only spark discharging,

Re. 26,090 Reissuecl Sept. 20, 1966 ice it is necessary to select thepulse with sufficiently short width. For this reason the melting away ofthe shortcircuit points with excessive energy is, in general,undesirable.

The use of a pluse of narrow width, as mentioned above, is sometimesaccompanied by the following disadvantageous result. That is, thedischarge energy is expended for only the formation of the crater moundand crater. The crater mound and crater which have, for this reason,been retained in the short-circuited condition with-out being meltedaway, are caused by the pulse of overwhelmingly excessive energy contentand subsequent melting away of the short-circuit points. This causes afurther increase in their crater mound portions and supplementarylengthening of said short-circuit points. In such a case, both theelectrode and work piece become completely short-circuited and preventfurther machining just as in the case of arc discharging.

In discharge processing, in general, it is necessary to secure andfollow up so as to maintain the gap between the electrode and work piececonstant at all times, regardless of the progress of the machining. If,as described above, short-circuiting occurs constantly, the preventionof short-circuiting of the electrode and work piece will become thepriority problem, and the maintenance of constant gap will becomeinsignificant. As counter measures, two methods are conceivable: themethod of temporarily stopping the supply of electric energy at the timeof short-circuiting, forcibly separating the electrode and work piece,and suppressing the development of discharge; and the method ofproviding the electrode servomechanism with precision, causing it tofollow up precisely and positively, and thereby preventingshort-circuiting.

In application of the former of the two methods mentioned above,operation may occur only when shortcircuiting has occurred over asubstantially long period of time (several cycles), Therefore, it cannotbe expected to operate with high sensitivity responding toshortcircuiting during only one cycle. Moreover, such a system requirescomparatively complex apparatus.

Also, in the application of the aforesaid second method, even if theelectrode servo-mechanism is provided with an extremely high degree ofprecision, if the expected frequency of the pulse repetitions is, say,from 500 to 1,000 kilocycles per second, it will be necessary to effectthe follow up of the electrode, also, within the range of 10* to 10*seconds, and it must be said that, because of inertia, the use of suchelectrode servo-mechanisms is practically impossible.

Moreover, a special relation exists between the length of spark gap andthe machining speed, and when the spark gap is of an appropriate length,a maximum machining speed is obtained, but when the spark gap is greateror smaller than said appropriate length, the machining speed is reduced.If short-circuiting is feared, and the electrode servo-mcchanism isdesigned so that the length of spark gap will be greater than theappropriate value, the frequency of discharge repetition will decrease,and this also contributes to the lowering of the machining speed. Or, ifan effort is made to maintain length of the spark gap at an appropriatevalue, the possibility of short-circuiting will constantly be present.In this case also, an ideal follow-up mechanism for blocking thispossibility cannot be hoped for because of the influence, as mentionedabove, of mechanical inertia and electrical time constant.

The above method has the disadvantage in that, even if the electrodeservo-mechanism is provided with a high degree of precision in order tosuppress the short-circuit ing as described above, the only result is toincrease the cost of construction, and the desired result cannot beattained.

In view of the foregoing points, it is an object of the presentinvention to provide a power supply system for electric dischargemachining wherein, after a primary, main discharging, a secondary,follow-up discharging is caused to take place, and the short-circuitingpoints are melted away without any conversion into arc discharging.

It is another object of this invention to provide an electric dischargecircuit which provides, in a short time interval, a secondary, follow-updischarge of high energy.

It is yet another object of this invention to provide a power supplysystem for electric discharge machining wherein the time at which thesecondary follow-up discharge commences is controlled in accordance withthe metal material to be used.

It is a further object of this invention to provide a power supplysystem for spark discharge machining wherein the time of flowing of thesecondary, follow-up discharge current may be controlled in accordancewith the kind of metal material to be used.

Such objects and other objects of this invention have been achieved, inone embodiment of this invention, by the apparatus wherein aninductance-capacity circuit which resonates at a relatively highfrequency is connected in parallel to a condenser for spark dischargemachining (for generating the main discharge pulses) connected acrossthe electrode and the work piece, so as to produce automatically, butonly when required, a secondary, follow-up discharge of a frequencywhich is high relative to the fundamental of the main discharge pulseenvelope. This secondary, high-frequency discharge circuit forms aclosed circuit through the spark gap only when short-circuiting iscaused to occur by the primary, main discharge, and supplies just enoughenergy to cause the melting away of the short-circuit points by jouleheat of the high-frequency current. Consequently, the high-frequencycurrent does not participate directly in normal machining due to thespark discharge. During a normal machining impulse the impedance changeacross the gap is insufiicient to induce high-frequency discharging, andthe energy is caused to remain in its stored state until the subsequentoccurrence of shortcircuiting. In view of the necessity of utilizing allmeans to supress the conversion into arc discharging, the apparatus ispreferably designed so that when a short-circuit condition inducessupplemental discharge from the high-frequency circuit thehigh-frequency discharge is dissipated and terminated, if possible,within one half cycle of the frequency of repetition of the principalspark discharge impulses.

The reasons for the selection of high-frequency current especially forthe secondary, follow-up discharge are as follows:

The first reason is that, if the current is a high-frequency current,this current can be obtained merely by connecting an extremely simple,inductance-capacity circuit in parallel to the condenser for pulsegeneration without the necessity of providing a special power source forhigh frequency current.

The second reason is that with this circuit almost no resistance existsin the circuit, and all of the energy stored in the condenser can beused for melting away the short-circuit points.

The third reason is that it is possible to establish the condition thatthrough the resistance (resistance determined by the degree of movementof the ions) of the discharge path which is created temporarily at thetime of spark discharging, the current is temporarily restrained by theinductances so that the energy in the condenser will not dischargedirectly, and once the short-circuit points have been created, it ispossible for the first time to build up the current from zero.

The said final reason is that the use of high-frequency current is anindispensable condition for the blocking of the conversion from sparkdischarging into arc dischargmg.

In another embodiment of this invention, a C-type resonant circuit isinductively coupled further to the inductance of the secondary,high-frequency discharge circuit; and the apparatus is so arranged thatthe eifective inductance of the circuit as a whole is reduced, and theeffective capacitance is increased. Thus, in the discharge energyexpressed by 1/2 CV increase of the term C is attained, and thevariation of the term V is compensated for.

In still another embodiment of this invention, the circuit for thesecondary, follow-up discharge is composed of a plurality ofparallel-connected, resonant circuits, each having a slightly differentresonant frequency, Then, at the time of short-circuit, the variousresonant circuits connected in parallel are simultaneously closed, andthe resultant discharge current assumes the form of a geometric envelopeof the combined forms of the discharge currents of the various resonantcircuits.

In a further embodiment of this invention, the secondary,high-frequency, discharge circuit is composed of a plurality ofdischarge circuits of different time constants so that in accordancewith the kind of metal material of the electrode and work piece anydischarge circuit having a special time constant may be selected.

Heretofore, a method wherein a high-frequency voltage is impresscdacross the electrode and work piece, and a direct-current voltage isfurther superimposed thereon to accomplish a spark discharge machininghas been proposed. By this method, since a direct-current bias isimposed on the high-frequency voltage, and only a half wave of a certainpolarity is caused to be impressed, the result is essentially no morethan merely forming highfrequency pulses of a certain polarity.Therefore, said method ditiers basically from the essence of the presentinvention.

In another spark discharge machining apparatus which has been proposedheretofore, pulses of a certain polarity are impressed across theelectrode and work piece and, simultaneously, a high-frequency voltageis superimposed thereon. Analysis of the discharge current of thisapparatus indicates that the imposed high-frequency current is modulatedby the discharge current of the condenser, that is, by the pulses of acertain polarity, and a high-frequency current is continuously caused toflow between the electrode and work piece. Consequently, thehigh-frequency current flows not only during the time ofshort-circuiting, but also during the other normal period of time andcontributes directly to the machining by spark discharging. On thispoint, also, the apparatus differs substantially from the essence of thepresent invention.

The unique features and advantages of this invention and the manner inwhich the foregoing objects may best be achieved will be more clearlyunderstood by reference to the following detailed description of a fewrepresentative embodiments of the invention when taken in connectionwith the accompanying drawings.

FIGURE 1 is an electrical connection diagram showing one embodiment ofthis invention, wherein a series resonant-type, secondary, follow-updischarge circuit is provided.

FIGURE 2 is a graphical diagram for describing the primary, maindischarge, voltage wave form and the secondary, follow-up dischargevoltage wave form occurring in an embodiment of the apparatus of theinvention.

FIGURE 3 is a graphical diagram showing the discharge voltage wave formin the case of an apparatus proposed heretofore, wherein high-frequencywaves are superimposed on pulses.

FIGURE 4 is an electrical connection diagram of another embodiment ofthe secondary, high-frequency discharge circuit according to theinvention.

FIGURE 5 is an electrical connection diagram of a modification of thecircuit shown in FIGURE 4.

FIGURE 6 is an electrical connection diagram of a further embodimentaccording to the invention, wherein a plurality of secondary,high-frequency discharge circuits, each having a slightly differentresonance frequency, is provided.

FIGURE 7 is a graphical diagram for describing the secondary, follow-updischarge voltage of the apparatus shown in FIGURE 6.

FIGURE 8 is a graphical diagram showing the primary, main discharge andsecondary, follow-up discharge voltage wave form of the apparatus ofFIGURE 6.

FIGURE 9 is an electrical connection diagram showing another embodimentof the secondary, follow-up discharge circuit corresponding to that ofFIGURE 6.

FIGURE is an electrical connection diagram showing a still furtherembodiment of this invention, wherein a switching device for selectivelyconnecting one of a plurality of secondary, follow-up discharge circuitsof different time constants is provided.

Throughout the above illustrations, like reference numerals or symbolsdesignate like or equivalent circuit elements.

Referring to FIGURE 1, the machining electric power supply source 1 isused for impressing electric pulse voltage across the discharge gapbetween an electrode 2 and a work piece 3 which is used as anotherelectrode. In the design of said power source 1 consideration has beengiven to the providing of control so that, at the time ofshort-circuiting of said gap, the output voltage of said power sourcewill be lowered abruptly. A condenser 4 for producing electric pulses ischarged by the output of said power source 1 through an inductor 5acting as a filter. The power source 1 is composed of an alternatingcurrent source 1a, a transformer 1b, a saturable reactor 1c having adirect-current exciting coil 1d, and a rectifier device 1c, saidexciting coil 1d being connected to the output side of said rectifierdevice. Accordingly, when the discharge gap between the electrode 2 andwork piece 3 is short-circuited, the current of the exciting coil 1d islowered abruptly, whereby the impedance of the reactor 1c becomes largeand the output voltage of the power source 1 is lowered abruptly.

It is a unique feature of the present invention that a series resonantcircuit 6 is connected across the electrode 2 and the work piece 3 andat the same time in parallel to a condenser 4 for producing pulses. Saidseries resonant circuit 6 is composed of an inductor 7 and a condenser8. The capacity of the condenser 8 is indicated by experimental resultsto be suitable when it is of the order of one-tenth of the capacity ofthe condenser 4 for producing pulses. In normal operation it may be seenthat condenser 4 may have a capacity varying from about 0.1 to 70microfarads dependent upon the speed of erosion and finish desired,while condenser 8 would have a capacity varying from about 0.01 tomicrofarads. When a short circuit occurs the charge held by condenser 4becomes instantaneously 0; however, the charge held by a condenser 8will fiow into the short circuit creating heat to burn, hence eliminatethe short circuit, at which instant discharges will recommence from theprimary circuit. The wave forms, or wave profiles, of the primary, maindischarge and secondary, follow-up discharge voltages at the time ofdischarge machining may be representcd graphically as shown in FIGURE 2.As indicated in this diagram, the voltage of pulse V produced from thecondenser 4 achieves the metal machining due to spark discharging undera certain period of cycle. If, as a supposition, short-circuiting occursin each cycle, the voltage of the secondary, follow-up discharge pulse Vwill be generated as shown in FIGURE 2, and, only when the voltage ofthe pulse V reaches a minimum value or a value in the vicinity thereof,the pulse V appears suddenly.

In the case of apparatus proposed heretofore wherein a high-frequencyelectric power source is connected in parallel with a condenser forproducing pulses, the voltage wave forms differ from those of FIGURE 2and are as shown in FIGURE 3. As is apparent from this diagram, thehigh-frequency electric voltage is constantly superimposed on the pulsevoltage, and a constant, highfrequency energy is imparted continuouslybetween the electrode and the work piece during the time of machining byspark discharging and also during the time of shortcircuiting.Consequently, the spark discharging is constantly accompanied by thepossibility of its conversion into an arc discharging.

In contrast, in the case of apparatus of the present invention, thehigh-frequency electric current due to the secondary, follow-updischarge is zero at the time of beginning of spark discharging andbuilds up from zero only after circuit closure due to short-circuiting,consequently, contributing in no way to the spark discharge machining.Accordingly, it is possible to prevent, completely, the conversion intoan arc discharging.

Moreover, in the case of conventional, discharge machining apparatus,wide gap between the electrode and work piece are used out of fear ofshort-circuiting, even at the sacrifice of machining speed. In contrast,in the case of the apparatus of the present invention, since it isalways possible to melt away the short-circuit points which createpossibilities of short-circuiting, it is possible to place the electrodeand the work piece as closely as possible to each other, and it ispossible to multiply the machining speed and, at the same time, toimprove the degree of. smoothness of the machined surface.

The secondary, follow-up discharge circuit need not be limited to aseries resonant circuit; it is possible to use a C-type resonantcircuit. The use of said C-type resonant circuit is illustrated inFIGURES 4 and 5, wherein parallel resonant circuits 9 and 10 areconnected to an inductor 7 of a series resonant circuit.

Moreover, the secondary, follow-up discharge need not always be ahigh-frequency electric current. As long as the short circuit points aremelted away by the joule heat, the use of pulses with narrow widths mayalso be recommended. However, it is not the best practice to supply thispulse from another electric power source because, although it ispossible to provide, especially an electric power source which will passa pulse for secondary, follow-up discharge for every interval of thepulse for the main discharge, this will not only complicate theapparatus needlessly, but also create problems in cost. Furthermore, theproviding of a separate electric power source for selectively producingpulses only at the time of short-circuiting is beyond consideration.

In view of the above points, the present invention further proposes anelectric power source for discharge machining provided with a pluralityof high-frequency resonant circuits, each with a slightly differentresonant frequency. One embodiment thereof is illustrated in FIGURE 6,wherein the secondary, follow-up discharge circuit 6 comprises a seriesresonant circuit composed of a reactor 7' and a condenser 8', a seriesresonant circuit composed of a reactor 7 and a condenser 8, and a seriesresonant circuit composed of a reactor 7" and a condenser 8", saidresonant circuits being connected in parallel, and having resonancefrequencies diifering slightly from one another. The wave forms of thehighfrequency discharge currents due to these circuits at the time ofshort-circuiting are as shown in FIGURE 7, and the envelope curve ofsaid forms is a damped oscillation of a rectangular Wave with positiveand negative polarity. If the apparatus is designed so that the dampingis effected abruptly and the following pulse for spark dischargemachining is impressed to the machine points at the time when the saidenvolpe curve converts to the negative side, the machining electricvoltage will be approximately as shown in FIGURE 8. Thus, it is possibleto produce pulses selectively at the time of short-circuiting with theuse of an extremely simple circuit, and moreover, it is possible to makesaid pulses have greater energy than that of any other wave form, thatis, to make pulses of rectangular wave form.

Furthermore, the composing of the secondary, followup discharge circuitwith the use of a plurality of resonant circuits has the followingadvantages. Even if the capacity of the condenser 8 is increased in aneffort to obtain a high energy from a single, highfrequency dischargecircuit as shown in FIGURE 1, the discharge current cannot be increasedin proportion to the capacity, and in general, a linear relation doesnot exist between said discharge current and capacity. Therefore, if thecapacity of the condenser 8 is divided, and distributed among theparallel circuits 7-8, 7'8', 7"8" as shown in FIGURE 6, the dischargecurrent will increase approximately lineanly in accordance with thenumber of parallel circuits, and results which could never be expectedfrom a single, high-frequency discharge circuit will be obtainable.

For this follow-up discharge circuit the same effect can be obtained, ofcourse, by connecting in parallel a plurality of parallel resonantcircuits 7,8', 7",8", 7", 8" as shown in FIGURE 9, in which condensers13, 13' and 13" are provided for suppressing direct current.

It has been determined from experimental results that if theabove-mentioned secondary, follow-up discharge energy instead of beingsupplied uniformly in an indiscriminate manner to any work to bemachined is varied according to the kind of metal of the work, goodresults will be obtainable. More specifically, even if the shortcircuitpoints are to be melted away there are many metals requiring a greatvariety of form of follow-up discharge such as depending on the kind ofmetal, those for which good results can be obtained by suitableretarding the instant of beginning of discharging of the secondary,follow-up discharge current; those requiring a considerably long periodof current flow, or those for which good results are obtained byadvancing, as much as possible, the instant of beginning of dischargingand, at the same time, holding the current fiow period to a short time.For example, in the case wherein the work to be machined is copper, goodresults are obtained by causing the secondary, follow-up discharge totake place after the elapse of a short time subsequent to the dischargeof the pulse for spark discharge machining; or, in the case of extremelyhard metals, it is preferable that the secondary, follow-up dischargetake place immediately after the completion of the spark discharge.

In view of the above-described points, the present invention provides apower-supply apparatus for discharge machining which is further socomposed as to enable the selection of the secondary, follow-updischarge circuit of the optimum time constant, in each case for themetal material of the work piece and the electrode.

Referring to FIGURE 10, the secondary, follow-up discharge circuit 6 iscomposed of high-frequency, resonant circuits 6, 6" and 6', each havinga different time constant. One end of each of these circuits isconnected to the electrode 2, and the other end thereof is connected toswitch taps 11', 11" and 11". The said switch taps are provided with achangeover switch 12, which enables adjustment, at will, in accordancewith the kind of metal use-d. Thus, each circuit is selectivelyconnected to the condenser 8', 8" or 8' for producing pulses, and theaforesaid object is achieved.

It is evident that, instead of the use of the abovedescribed switchingmechanism, the use of variable inductors or variable condensers in thehigh-frequency, resonant circuits would enable the accomplishment of thesame object.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to the details described herein and to the embodimentsillustrated in the accompanying drawings except as set forth in theappended claims.

I claim as my invention:

[1. Electric discharge machining apparatus comprising, in combinationwith a machining electrode and base means adapted to present a metalworkpiece in electric spark discharge relationship to said electrodeseparated therefrom by a spbank gap, a voltage source including astorage capacitance connected between said electrode and workpiece andmeans to charge said capacitance to predetermined machining voltagethrough an impedance, thereby to create intermittent primary sparkdischarge impulses between the electrode and workpiece, the transientvoltage across said gap dropping by a substantially greater amount whena short circuit develops across said gap during a discharge impulse thanduring other, normal discharge impulses, and separate circuit means alsoconnected across said gap and selectively responsive to said greatervoltage drop to apply secondary spark discharge voltage impulses acrossthe electrode and workpiece substantially only in response to suchgreater voltage drops, thereby to clear the short-circuit condition bythe added discharge energy of said secondary impulses without addingsuch energy to normal discharge impulses] [2. The apparatus defined inclaim .1, wherein the separate circuit means comprises a resonantcircuit having a natural frequency which is at least several times theprimary impulse basic frequency] [3. The apparatus defined in claim 1,wherein the separate circuit means comprises a plurality of separateresonant circuits having respectively different resonance frequencieseach of which is at least several times the primary impulse basicfrequency, and switch means operable selectively to connect saidresonant circuits individually across said electrode and workpiece] [4.The apparatus defined in claim 1, wherein the impedance is variable andthe power source includes means responsive to variations in spark gapvoltage to increase the value of said impedance in response to thedecreased spark gap voltage during a short circuit across the gap] [5.The apparatus defined in claim 4, wherein the separate circuit meanscomprises L-C resonant circuit means having a natural frequencysubstantially higher than the primary spark discharge frequency] [6. Anelectric power supply apparatus for electric discharge machining of aworkpiece by a machining electrode, said apparatus comprising a storagecondenser connected across the electrode and workpiece, means to chargethe storage condenser recurringly to a discharge voltage producingperiodic primary spark discharge impulses between the electrode andworkpiece, said latter means including an alternating current source, arectifier energized by said source and having an output connected to thestorage condenser, and a normally saturated saturable reactor interposedbetween the source and rectitier and having a control winding connectedto be energized by said rectifier output, the voltage drop across theworkpiece and electrode during a primary discharge impulse be-comingabnormally great during a short-circuit condition therebetween, and arelatively high-frequency resonant circuit also connected across saidelectrode and workpiece and operable selectively in response to saidabnormally great voltage drops to generate secondary follow-updischarges therebetween for clearing such shortcircuit condition] [7. Anelectric power supply apparatus for electric discharge machining of aworkpiece by a machining electrode, said apparatus comprising a storagecondenser connected across the electrode and workpiece, means to chargethe storage condenser recurringly to a discharge voltage producingperiodic primary spark discharge impulses between the electrode andworkpiece, said latter means including an alternating current source, arectifier energized by said source and having an output connected to thestorage condenser, and a normally saturated saturable reactor interposedbetween the source and rectifier and having a control winding connectedto be energized by said rectifier output, and means to clear ashort-circuit condition between the electrode and workpiece, comprisinga plurality of relatively high-frequency resonant circuits of slightlyditferent natural frequencies, respectively, also connected in parallelacross said electrode and workpiece and operable in response to theelectrical transient which develops during a short-circuit therebetweento generate secondary follow-up discharges therebetween in response tosuch primary discharges] [8. Electric spark discharge machiningapparatus comprising, in combination with an electrode and workpieceseparated by a discharge gap, means connected across the electrode andworkpiece to induce recurrent primary spark discharge impulses throughsaid gap each attended normally by predetermined gap voltage transients,and separate circuit means connected across the electrode and workpiecenormally unresponsive to said gap voltage transients, said separatecircuit means being selectively responsive to abnormal voltagetransients attending short circuiting of the gap during a primaryimpulse, and being operable thereby to induce secondary dischargesthrough said gap to clear the short-circuit condition] [9. Thecombination defined in claim 8, wherein the separate circuit meanscomprises at least one resonant circuit having a time constant which isa small fraction of the duration of the primary discharges] [10. Inelectric spark discharge machining by producing a succession of primaryspark discharge impulses across a spark gap between an electrode andworkpiece, the method of controlling machining voltage across the sparkgap comprising the steps of reourringly increasing the machining voltageto a value which recurringly initiates a primary spark discharge acrossthe gap succeeded immediately by a normal drop of gap voltage to a valuenormally suflicient to terminate the spark discharge, and impressing asecondary, oscillatory discharge voltage across the gap selectively inresponse to an abnormally abrupt reduction of gap voltage inherentlyoccurring under short circuiting of the gap which prevents terminationof an individual primary discharge, whereby said secondary dischargevoltage adds energy to the discharge sufficient to clear the shortcircuit] 11. The process of electrical discharge machining of aconductive workpiece across a dielectric filled gap by an electrode toolcomprising the steps of passing a series of machining pulses ofrelatively low frequency and long duration across the gap, impressing aseries of relatively high frequency, short duration pulses ofsuccessively diminishing amplitude across the gap prior to the normalcompletion of each machining pulse and terminating said higher frequencypulses prior to the initiation of the next following machining pulse,the time duration of each short duration pulses being a small fractionof that of said machining pulses.

12. The process of electrical discharge machining of a conductiveworkpiece across a dielectric filled gap by an electrode tool comprisingthe steps of passing a series of machining pulses of relatively lowfrequency and long duration across the gap, impressing a series of shortduration, damped, oscillatory pulses of successively diminishingamplitude across the gap at a frequency at least several times thefrequency of said machining pulses prior to the normal completion ofeach machining pulse and terminating said oscillatory pulses prior tothe initiation of the next following machining pulse, the time durationof each short duration pulse being a small fraction of that of saidmachining pulses.

13. The process of electrical discharge machining of conductiveworkpiece across a dielectric filled gap by an electrode tool comprisingthe steps of passing a series of machining pulses of relatively lowfrequency and long duration across the gap, impressing across the gap atthe time of sharp drop in voltage across the gap and prior to normalcompletion of each machining pulse a series of short duration, damped,oscillatory pulses of higher frequency than said machining pulses, andterminating said oscillatory pulses prior to initiation of the nextfollowing machining pulse, the time duration of each short durationpulse being a small fraction of that of said machining pulses.

14. The process of electrical discharge machining of a conductiveworkpiece across a dielectric filled gap by an electrode tool comprisingthe steps of passing a series of machining pulses of relatively lowfrequency and long duration across the gap, and impressingsimultaneously across the gap a plurality of oscillatory voltagewaveforms of slightly varying frequencies, all in excess of themachining pulse frequency to provide a: compo ite, damped, oscillatorywaveform between successive machining pulses said composite waveformsuperimposed upon the latter portion of each preceding machining pulse.

15. The process of electrical discharge machining of a conductiveworkpiece across a dielectric filled gap by an electrode tool comprisingthe steps of passing a series of machining pulses of relatively lowfrequency and long duration across the gap and modulating each of saidmachining pulses with a series of superimposed, high energy, shorterduration, higher frequency pulses for a time period preceding its normalcompletion and falling within one half cycle of its frequency ofrepetition, the time duration of each shorter duration pulse being asmall fraction of that of said machining pulses.

16. The process of electrical discharging machining of a conductiveworkpiece by an electrode tool across a dielectric coolant filled gapcomprising the steps of passing a series of machining pulses ofrelatively low fre quency and long duration across the gap, andimpressing simultaneously across the gap a plurality of oscillatoryvoltage waveforms of slightly difiering frequencies, all in excess ofthe machining pulse frequency, to provide a composite, damped,oscillatory waveform of positive and negative polarity betweensuccessive machining pulses, said composite waveform superimposed uponthe latter portion of each preceding machining pulse.

17. The process of electrical discharge machining of a conductiveworkpiece by an electrode tool across a dielectric coolant filled gapcomprising the steps of passing a series of machining pulses ofrelatively low frequency and long duration across the gap, andimpressing simultaneously across the gap a plurality of oscillatoryvoltage waveforms of slightly difiering frequencies, all in excess ofthe machining pulse frequency, toprovide a composite, damped oscillatorywaveform between successive machining pulses, said composite waveformspaced by a short time period from each preceding machining pulse.

18. The process of electrical discharge machining of a conductiveworkpiece by an electrode tool across a dielectric coolant filled gapcomprising the steps of passing a seies of machining pulses ofrelatively low frequency and long duration across the gap, andimpressing simultaneously across the gap a plurality of oscillatoryvoltage waveforms of slightly differing frequencies, all in excess ofthe machining pulse frequency, to provide a composite, damped,oscillatory waveform between successive machining pulses, said compositewaveform immediately following the preceding machining pulse.

19. The process of electrical discharge machining of a conductiveworkpiece by an electrode tool across a dielectric filled gop comprisingthe steps of charging a gap capacitor to a voltage sufficient toinitiate a gap breakdown pulse and, subsequent to gap breakdown,impressing simultaneously across the gap a plurality of oscillatoryvoltage waveforms of relatively high, slightly differing frequencies, toprovide a composite, damped, oscillatory waveform across the gap.

20. The process of electrical discharge machining of a conductiveworkpiece by an electrode tool across a dielectric filled gap comprisingthe steps of charging a gap capacitor to a voltage suflicient toinitiate a gap breakdown pulse and, subsequent to gap breakdown,impressing simultaneously across the gap a plurality of oscillatoryvoltage waveforms of relatively high, slightly difiering frequencies, toprovide a composite, damped, oscillatory waveform across the gap, saidcomposite waveform spaced by a short time period from said gap breakdownpulse.

21. The process of electrical discharge machining of a conductiveworkpiece by an electrode tool across a dielectric filled gap comprisingthe steps of charging a gap capacitor to a voltage sufiicient toinitiate a gap breakdown pulse and, subsequent to gap breakdown,impressing simultaneously across the gap a plurality of oscillatoryvoltage waveforms of relatively high, slightly difiering frequencies, toprovide a composite, damped, oscillatory waveform across the gap, saidcomposite waveform immediately following said gap breakdown pulse.

22. The process of electrical discharge machining of a conductiveworkpiece by an electrode tool across a di- References Cited by theExaminer The following references, cited by the Examiner, are of recordin the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,880,374 3/1959 Mulder 315205 2,895,080 7/1959Branker 315205 2,951,969 9/1960 Matulaitis et al. 21969X 3,089,0595/1963 Porterfield et al. 21969 X RICHARD M. WOOD, Primary Examiner.

RALPH G. NILSON, Examiner.

L. D. BULLION, R. F. STAUBLY, Assistant Examiners

